Dual connectivity is one of the features being standardized within the umbrella work of small cell enhancements within 3GPP Rel-12. Dual connectivity is a feature that allows a user equipment (UE) to simultaneously receive and transmit to at least two different network points. The two different network points are usually denoted as master eNodeB (MeNB) and secondary eNodeB (SeNB). MeNBs serve a master cell group (MCG), and SeNBs serve a secondary cell group (SCG). It is assumed that the radio resource control (RRC) protocol, which is responsible for configuring the UE, is terminated within the MeNB. While the UE receives RRC control signaling via the MCG, it may receive user data via both MCG and SCG.
FIG. 1 is a block diagram of a protocol architecture in dual connectivity. The currently envisaged protocol architecture for Rel-12 supports three types of radio bearers within a MeNB 10 and a SeNB 20. The three types of radio bearers are MCG bearer 30, SCG bearer 40, and split bearer over MCG and SCG 50. MCG bearer 30 includes packet data convergence protocol (PDCP) 32 and radio link control (RLC) 34. SCG bearer 40 includes PDCP 42 and RLC 44. Split bearer 50 includes PDCP 52, RLC 54 and RLC 56. In the case of the split bearer 50, user data that is received by PDCP 52 is split via both RLC 54 of MeNB 10 and RLC 56 of SeNB 20. It is envisaged that some PDCP packet data units (PDUs) may be transmitted to the MeNB RLC 54 and some may be transmitted to the SeNB RLC 56. Due to the transmission delay of the backhaul link between MeNB 10 and SeNB 20, and different radio link conditions between MeNB 10 and the UE and SeNB 20 and the UE, the receiving PDCP protocol entity receives PDUs out of order. To provide in-order delivery to higher layers, PDCP needs to buffer received PDUs and reorder them, based on a reordering window and by using a reordering timer. A typical reordering window has the size of half the sequence number space.
Feedback between MeNB 10 and SeNB 20 may be required for the split bearer operation in dual connectivity to balance the dataflow between MeNB 10 and SeNB 20. The feedback allows MeNB 10 to determine how much data it needs to send to SeNB 20 so that the SeNB transmitter queue is neither underutilized nor overloaded. Furthermore, feedback for the PDCP transmitting entity in MeNB 10 from SeNB 20 to MeNB 10 is necessary to make sure that for split bearers, MeNB 10 does not bring more than half of the PDCP sequence number space in flight. A PDCP receiver based on a reordering window of half the sequence number space is not able to correctly handle more data in flight than half the sequence number space. If MeNB 10 brings more than half the PDCP sequence number space in flight, problems such as hyper frame number (HFN) de-sync may occur. There is a need for a feedback mechanism that fulfills both these requirements and allows MeNB 10 to determine how much of PDCP SN space is currently in flight in order to advance the transmission window.